1. Field of the Invention
The present invention relates to a bearing structure used in a rotating machine such as a motor and to a rotating machine having the bearing structure.
2. Description of Related Art
A motor shown in FIG. 5 has been used as a fan motor for an airconditioner unit in an automobile. A motor 51 is composed of a housing 52, an rear end frame 53 closing an open end of the housing 52 and a rotor 54 having a shaft 55 rotatably supported in the housing 52 and the rear end frame 53. The rotor shaft 55 is rotatably supported by both bearing bushes 56, each having a bearing hole 56a. One of the bearing bushes 56 located in the front end is supported in a cup portion of the housing 52 and held therein by a bearing retainer 57. The other bearing bush 56 located in the rear end is supported in a cup portion of the rear end frame 53 and held therein by another retainer 57. Outer peripheral end corners of the bearing bush 56 are rounded, and the cup portion and the retainer 57 holding the bearing bush also have a round surface to contact the rounded corners of the bearing bush. Therefore, the bearing bush 56 can rotate a little in the cup portion to automatically align with the axis of the rotor shaft 55 when the bearing bush receives external force such as vibration. This is known as "self-alignment."However, a position where the shaft actually contacts the bearing hole and is supported ("supporting position") differs depending on delicate condition of the self-alignment. Accordingly, a distance between supporting positions of both bearing bushes varies in a range of two times of the length of the bearing bush. A natural bending vibration frequency of the shaft 55 also varies according to the variation of the distance between supporting positions. If the natural vibration frequency of the shaft becomes close to a basic frequency of rotational speed of the motor, the shaft and the motor vibrate in resonance, and resonance noises will be generated.
Another example of this kind of motor is shown in JP-U-5-69435, in which a third bearing is disposed between both end bearings to suppress an abnormal vibration of a shaft caused by bending of the shaft. Also, a relief bore having a tapered surface is disposed at an outer end of a bearing hole of the end bearing, so that the shaft does not contact the bearing hole at a portion where the relief bore is formed. However, since no self-alignment structure is employed in the motor, vibration noises will be generated when a misalignment between the rotational axis and the bearing holes exists.